Boron nitride film and process for preparing same

ABSTRACT

A boron nitride (BN) film is disclosed having excellent electrical insulating and heat conduction characteristics and capable of acting as seeds for epitaxially growing a semiconductor film thereon which has a crystallizability sufficient to incorporate a semiconductor element therein. The BN film is formed by the growth on a substrate in a manner to be preferentially orientated to a predetermined axis thereof. A process for preparing such film is also disclosed.

This is a division, of application Ser. No. 584,403, filed Feb. 28,1984, now U.S. Pat. No. 4,565,741.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a boron nitride film and a process forpreparing the same, and more particularly to a boron nitride film ofexcellent electrical insulating and heat conduction characteristicsformed on a substrate in a manner to be preferentially oriented to apredetermined axis thereof which is suitable for use as an intermediateinsulating layer in a three-dimensional semiconductor device to allowsemiconductor layers each having a semiconductor element incorporatedthereinto to be threedimensionally laminated to accomplish the highdensity packaging of the semiconductor device or as an intermediateinsulating layer in a semiconductor device for large power.

2. Description of the Prior Art

Recently, researches have been made in the threedimensional packaging ofa semiconductor device wherein semiconductor chips and/or layers aresuperposed to form one semiconductor element, with the high integrationof a semiconductor element. However, it is required to solve variousproblems resulting from the structural features of a semiconductor inorder to realize such three-dimensional packaging.

One of the problems is that it is difficult to form, on an intermediateinsulating layer insulating interlayer a semiconductor device, asemiconductor layer having a crystallizability sufficient to incorporatea semiconductor element therein. When an insulating layer has beendeposited on a semiconductor base plate of which the surface hasroughness formed thereon due to a semiconductor element which has beenalready incorporated into the semiconductor base plate, it is impossibleto grow a semiconductor layer of a good crystallizability on theinsulating layer because the surface of the insulating layer exhibits anirregular state.

Another important problem is how to attain insulation betweensemiconductor layers of a semiconductor device and how to improve heatdissipation of the device. An electrical insulator is generally inferiorin heat conduction characteristics, and electrical insulating propertiesand heat dissipation characteristics are contrary to each other. Thus,if both electrical insulating properties and heat dissipationcharacteristics of a semiconductor device are improved giving carefulconsideration to the selection of materials, the manufacturing of asemiconductor device and the like, the high integration and high densitypackaging of a semiconductor device can be significantly promoted.

The inventors proposed a beryllium oxide (BeO) and a method of preparingthe same, and a semiconductor device using the BeO film in view of theforegoing problems of the prior art while taking notice of the fact thatthe use of a BeO film having excellent electric insulating propertiesand heat conduction characteristics as an insulating layer betweensemiconductor layers of a semiconductor device and the growth of atleast one of the semiconductor layers on the BeO film allow thesemiconductor layer to have a crystallizability sufficient toincorporate a semiconductor element thereinto. However, the prior art isoften encountered with difficulty in the formation of such BeO film.

Now, attention is paid to boron nitride (hereinafter referred to as"BN") in view of the fact that it has the most excellent electricalinsulating properties and heat conduction characteristics next to BeO. ABN film has been conventionally formed on a semiconductor layeraccording to a chemical vacuum deposition (CVD) process or a sputteringprocess. However, all the so-formed BN films are amorphous. Although,the film has a thermal conductivity substantially equal to stainlesssteel, it makes the epitaxial growth of a semiconductor layer thereonhaving a crystallizability sufficient to incorporate a semiconductorelement thereinto substantially impossible.

BRIEF SUMMARY OF THE INVENTION

The present invention has been made in view of the foregoingdisadvantages of the prior art.

Accordingly, it is an object of the present invention to provide a BNfilm of excellent electrical insulating properties and heat conductioncharacteristics which is capable of exhibiting thermal conductivityabout three times as much as the conventional amorphous BN film, andacting as an orientation film utilized as seeds for epitaxially growinga semiconductor layer thereon even if the BN film has been formed on anamorphous substrate or a substrate having roughness formed thereon.

It is another object of the present invention to provide a BN film whichis capable of being used as an insulating layer between semiconductorlayers of a three-dimensional semiconductor device to significantlypromote the high integration and high density packaging of the device.

It is a further object of the present invention to provide a BN filmwhich is capable of being used as an insulating layer between a heatdissipation plate and a semiconductor layer in a semiconductor devicefor large power to provide the device with excellent heat dissipationcharacteristics while keeping high electrical insulating properties.

It is still a further object of the present invention to provide aprocess for preparing a BN film having such excellent advantages asdescribed above.

In accordance with the present invention, there is provided a boronnitride film grown on the surface of a substrate in a manner to bepreferentially orientated to a predetermined axis thereof. In accordancewith another aspect of the present invention, there is provided aprocess for preparing a boron nitride film comprising the steps ofheating boride charged in a closed-type crucible having at least oneinjection nozzle to form boron vapor; ejecting said boron vapor throughsaid nozzle into a nitrogen atmosphere having a pressure below 10⁻² Torrand ionizing at least a part thereof; and impinging said ionized boronvapor on the surface of a substrate together with nitrogen to form aboron nitride film on said substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects and many of the attendant advantages of the presentinvention will be readily appreciated as the same becomes betterunderstood by reference to the following detailed description whenconsidered in connection with the accompanying drawings, wherein:

FIG. 1 is a schematic view showing one example of an apparatus suitablefor use in the preparation of a BN film according to the presentinvention;

FIGS. 2 to 7 are schematic sectional views respectively showing first tosixth semiconductor devices in each of which a BN film is used as anintermediate insulating layer;

FIG. 8 is a graphical representation showing a result of X-raydiffraction analysis of a BN film formed on a monocrystalline Sisemiconductor layer;

FIG. 9 is a graphical representation showing electrical conductivity ofa BN film according to the present invention measured near a roomtemperature; and

FIG. 10 is a schematic sectional view showing a further semiconductordevice in which a BN film of the present invention is used as aninsulating interlayer.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Now, the present invention will be described hereinafter by way ofexample with reference to the accompanying drawings.

First, an example of an apparatus will be described with reference toFIG. 1 which is adapted to prepare a BN film of the present inventionaccording to an ionized cluster beam deposition process (hereinafterreferred to as "ICB") process.

The apparatus shown in FIG. 1 includes a closed-type crucible 1 havingat least one injection nozzle 2 of about 0.5 to 2.0 mm in diameterformed at the upper portion thereof. The nozzle 2 is desirably formed tohave an aspect ratio less than 1 by rendering the dimension along theaxial direction thereof smaller than the diameter. The crucible 1 ischarged with boron oxide (B₂ O₃) 3 in the form of powder, flake orpellet.

The apparatus also includes a heating device 4 provided around thecrucible 1. In the example illustrated in FIG. 1, the heating device 4is the electron impact type which comprises a coil filament arrangedaround the crucible 1 and adapted to emit electrons when heated, and apower source (not shown) for applying positive potential to the crucible1 to accelerate electrons emitted from the filament, so that electronsare permitted to impinge on the outer surface of the crucible 1 to heatit.

Alternatively, the crucible 1 may be heated by resistance heating,radiation heating or a combination thereof. The resistance heating isapplicable when the crucible 1 is formed of a conductive material, andis carried out by providing the upper and lower portions of the crucible1 with specific terminals and flowing a large current into the cruciblevia the terminals under a low voltage, to thereby heat the entirecrucible 1. The radiation heating is attained by arranging a heateraround the crucible 1 to heat it.

In addition, the apparatus includes a heat shielding plate 5 and anionization chamber 6. The ionization chamber 6 comprises a mesh-likeanode 7 formed into a cylindrical shape, a rectangular cylindrical shapeor a parallel-shape plate so as to surround a path for B₂ O₃ vapordescribed hereinafter and an electron emitting filament 8 and a shieldplate 9 which are arranged around the anode 7.

Further, the apparatus includes a holder 10 for holding thereon asubstrate 11 on which a BN film is to be deposited and a shutter 12serving to prevent the impingement of boron (B) or oxygen (O) onto thesubstrate 11 when the impingement is not desired. The substrate 11 maybe a monocrystalline semiconductor substrate such as, for example,monocrystalline silicon, a metal substrate, an amorphous insulatingsubstrate such as a glass substrate, or the like, as desired.

Reference numeral 13 designates a gas supply pipe provided with at leastone nozzle 14. In the illustrated example, the pipe 13 is disposed toallow the nozzle 14 to be arranged in proximity to the injection nozzle2 of the crucible 1. Alternatively, it may be provided in a manner toposition the nozzle 14 adjacent to the substrate 11.

Between the substrate 11 and the ionization chamber 6, an accelerationelectrode 15 may be arranged as desired. The acceleration electrode 15is applied thereto potential negative with respect to the crucible 1from a power source (not shown) to accelerate B₂ O₃ vapor ionized in amanner as described hereinafter.

Furthermore, the apparatus may have a heating means (not shown) arrangedadjacent to the substrate 11 as required which acts to heat thesubstrate.

The apparatus constructed as described above is received in a vacuumcasing (not shown) and supported therein by a suitable support means.The vacuum casing is connected to an evacuation apparatus (not shown) sothat a high vacuum atmosphere of 10⁻⁶ Torr or less is formed therein.The vacuum casing is also adapted to have a reactive gas G such asforming gas introduced therein through the gas supply pipe 13. Theforming gas introduced in the casing may be, for example, a mixed gasconsisting of 90 parts of nitrogen gas and 10 parts of hydrogen gas.

Now, a BN film of the present invention will be described which isprepared using the apparatus described above.

First, the crucible 1 is charged with boron oxide (B₂ O₃) 3 formed intoa suitable shape and the vacuum casing is evacuated to a pressure of10⁻⁶ Torr or less. Then, a mixture G of nitrogen gas and hydrogen gasmixed with a ratio of 90 : 10 is introduced in the vacuum casing throughthe gas supply pipe 13 to keep it at a pressure below 10⁻² Torr,preferably as low as 3.5×10⁻⁴ Torr.

Then, the heating device 4 is actuated to heat boron oxide 3 in thecrucible at a temperature between 1350° C. and 1450° C. to form vapor 3aof the boron oxide, which is ejected through the injection nozzle 2 ofthe crucible 1 to the exterior of the crucible 1 having a nitrogenatmosphere due to the difference in pressure between the interior andexterior of the crucible.

The ejected vapor 3a has kinetic energy imparted thereto whichcorresponds to its injection velocity, to thereby form a vapor stream 3bflowing toward the substrate and vapor-like boron (B) forms clusterscomprising a large aggregate of boron atoms loosely coupled together byvan der Waals forces by utilizing a supercooled state attained due tothe adiabatic expansion at the time of the ejection from the nozzle. Theformation of such clusters contributes to an improvement in theionization efficiency in the ionization chamber 6 and the formation of aBN film 16 of high quality on the substrate 11.

The vapor stream 3b having kinetic energy imparted thereto as describedabove enters the ionization chamber 6, in which at least a part of thevapor is ionized. More particularly, in the ionization chamber 6,electrons emitted from the filament 8 heated to a high temperature areaccelerated by voltage of 100 to 1000 V applied between the filament 8and the meshy anode 7 to impinge on the vapor stream 3b to allow atleast a part of the vapor to be ionized. In this instance, when thevapor stream 3b ejected from the nozzle 2 forms clusters, one of atomsforming the cluster is ionized due to the impingement of electronsthereon to form a cluster ion. Also, a part of the mixed gas G ejectedfrom the nozzle 14 reaches the ionization chamber to be ionized thereindue to electron impingement.

The ionized clusters are transported toward the substrate 11 togetherwith neutral clusters and the mixed gas G introduced in the path ofclusters, and are permitted to impinge on the surface of the substrate11 when the shutter 12 is opened, so that the clusters are separatedinto the individual atoms due to a surface migration effect to allow theatoms to diffuse on the surface of the substrate, to thereby form a BNfilm 16 on the substrate 11. After such deposition, the BN film 16 thusformed is subjected to annealing at a temperature of about 350° C. in avacuum atmosphere for 1 hour to remove moisture from the BN film 16which is formed by a reaction of hydrogen contained in the mixed gas Gwith oxygen of the boron oxide.

The vapor stream 3b impinging on the substrate 11, as described above,is ionized at least partially while it passes through the ionizationchamber 6, and the electrical field of the ion effectively acts on theinitial stage of the film formation on the substrate 11. Morespecifically, the electrical field of the ion promotes the formation ofseeds of crystal growth and further effectively acts on the coalescenceof atoms into an island region about the so-formed seeds. In addition,such ionization has another effect of promoting the reaction between Band N to allow the formation of a BN film having a stoichemicalcomposition and a good crystallizability.

Further, the vapor stream 3b is adapted to impinge on the substrate 11with kinetic energy imparted thereto at the time of the ejection. Thus,the formed BN film 16 has an excellent quality, because such incidentenergy contributes to the formation of a BN film which has a goodadhesion with respect to the substrate 11, the increase in packingdensity of the BN film and the improvement in crystallizability of theBN film. This is for the reason that the film 16 is preferentiallyorientated to a predetermined axis thereof or the C-axis even if thesubstrate 11 has roughness somewhat formed on the surface thereof orexhibits an amorphous property, and when the substrate 11 ismonocrystalline, the epitaxial growth of BN is accomplished on thesubstrate 11 to form the film 16 of a monocrystal regulated by thecrystallographic axis of the substrate 11.

Furthermore, the adjustment of acceleration voltage applied to the anode7 to vary an ionization current derived from the filament 8 allows thedegree of ionization of the vapor stream 3b to be controlled, so thatthe BN film 16 deposited on the substrate 11 may be further controlledin various properties such as crystallizability.

The embodiment illustrated, as described above, is adapted to eject thevapor 3a to the exterior of the crucible 1 utilizing the difference inpressure between the interior and exterior of the crucible to impartkinetic energy to the vapor 3a. However, the vapor stream 3b may beaccelerated by electrical field after at least a part thereof isionized, so that additional kinetic energy may be imparted to the vaporstream 3b to further carry out the control and/or improvement ofcrystallizability of the BN film 16.

More particularly, the application of voltage negative with respect tothe crucible 1, for example, the application of acceleration voltage ofabout 0 to 10 kV to the acceleration electrode 15 accelerates particlesof the vapor stream 3b ionized during the passing through the ionizationchamber to permit kinetic energy to be imparted to the ionized particle,so that the energy may effectively act to improve and/or control thecrystallizability of the film 16 at the time of the deposition tocontribute to the formation of the film 16 having a further excellentquality. In particular, such construction, when a monocrystallinesubstrate such as a silicon monocrystalline substrate is used as thesubstrate 11, exhibits an excellent advantage of effectively controllingcrystallizability of the BN film 16 by varying acceleration voltage tobe applied to the acceleration electrode 15 in view of the relationshipwith the crystal face of the substrate 11. Also, it is a matter ofcourse that the energy applied to the particles ionized by theelectrical field effectively serves to improve adhesion of the film withrespect to the substrate 11.

Alternatively, when the holder 10 is formed of a conductive material,such acceleration voltage may be applied between the holder 10 and thecrucible 1 without independently providing the acceleration electrode15.

The BN film formed in the manner as described above is adapted toeffectively act as an intermediate insulating layer or insulatinginterlayer in a semiconductor device of a three-dimensional structure.Some semiconductor devices into which a BN film of the present inventionis incorporated in various manners to act as an insulating interlayerwill be described with reference to FIGS. 2 to 7.

FIG. 2 shows a first semiconductor device of such type, which is formedas described hereinafter. First, a monocrystalline semiconductor such asa monocrystalline Si, GaP, GaAs or the like is deposited on aninsulating substrate 101 to form a lower semiconductor layer 102. Then,a semiconductor element is incorporated into the lower semiconductorlayer 102 utilizing conventional semiconductor element formingtechniques widely known in the art to form a base 110, which is mountedon the holder 10 of the apparatus shown in FIG. 1.

Then, a BN film 116 acting as an insulating layer is deposited on thelower semiconductor layer 102 according to the ICB process using theapparatus of FIG. 1 operated under the deposition conditions thatacceleration voltage Va applied to the acceleration electrode 15 andelectronic current Ie for ionization are set at 0.5 kV and 100 mA,respectively. The BN film 116, as described above, is grown in a mannersuch that the crystallographic axis C₁ is preferentially orientated tothe C-axis, even when a semiconductor element is incorporated into thesurface portion of the lower semiconductor layer 102 to the degreesufficient to lose the flatness of the surface. Thus, athree-dimensional film semiconductor device of the multilayer type canbe prepared by epitaxially growing an upper semiconductor layer 103 of amonocrystal on the BN film 116 according to an ion beam depositingprocess utilizing such orientation of the BN film and then incorporatinga semiconductor element into the upper semiconductor layer 103.

Alternatively, the semiconductor device shown in FIG. 2 may be preparedin a manner such that a material such as a rock salt which is easy todissolve in a solvent such as water and form a cleavage plane is used asa material for a substrate 101. A lower semiconductor layer 102, a BNfilm 116 and an upper semiconductor layer 103 are then deposited on thebase plate 101 in order according to a suitable procedure such as an ionbeam depositing process, an ICB process or the like and the base plate101 is dissolved to expose the lower surface of the lower semiconductorlayer 102 through which a semiconductor element is to be incorporatedinto the layer 102. In this instance, the BN film 116 is formed as amonocrystalline film because it is regulated by the crystallographicaxis of the lower semiconductor layer 102 to be epitaxially grown.

Also, in the first semiconductor device of FIG. 2, the uppersemiconductor layer 103 is formed by growing a monocrystalline film ofSi. Alternatively, the layer 103 may be formed of GaAs, GaSb, GaP, InSbor the like.

A second semiconductor device in which a BN film according to thepresent invention is used as an intermediate insulating layer is shownin FIG. 3, which is prepared by first depositing a first BN film 216acting as an insulating layer on a substrate 201 of an amorphousmaterial such as glass, ceramic or the like according to the ICB processunder the same deposition conditions as the first semiconductor device,as shown in FIG. 3, wherein the crystallographic axis C₂ of the BN film216 is preferentially orientated along the C-axis (002). Then, on thefirst BN film 216 is deposited a lower semiconductor layer 202 ofmonocrystalline Si according to the ICB process which is regulated bythe crystallographic axis of the BN film 216 to be epitaxially grown.Subsequently, a second BN film 226 is formed on the upper surface of thelower semiconductor layer 202 which is regulated by the crystallographicaxis of the lower semiconductor layer 202, and then an uppersemiconductor layer 203 of, for example, monocrystalline GaAs isdeposited on the second BN film 226, according to the ICB process, whichis regulated by the crystallographic axis of the film 226 to beepitaxially grown.

Thus, it should be noted that the second semiconductor device has anadvantage that a desired material may be used for the base plate 201irrespective of its insulating properties or conductivity.

FIG. 4 shows a third semiconductor device having a BN film of thepresent invention used therein as an insulating layer. The semiconductordevice comprises a lower semiconductor layer 302 formed of amonocrystalline Si semiconductor wafer, a BN film 316 acting as aninsulating layer which is deposited on the (111) plane of themonocrystalline Si in the substantially same manner as the first andsecond semiconductor devices, and an upper semiconductor layer 303 of aSi film deposited on the upper surface of the BN film 316 which isregulated by the crystallographic axis of the BN film 316 to beepitaxially grown.

FIG. 5 shows a fourth semiconductor device wherein a BN film accordingto the present invention is interposed as an insulating layer betweensemiconductor layers. The semiconductor device of FIG. 5 is prepared inthe following manner. First, a lower semiconductor layer 402 is formedof a monocrystalline Si semiconductor wafer in the substantially samemanner as the device shown in FIG. 4, the layer 402 acting as asubstrate. Then, a first BN film 416 serving as an insulating layer isdeposited on the lower semiconductor layer 402 in a manner such that itis regulated by the crystallographic axis of the lower semiconductorlayer 402. On the first BN film 416 is then formed a first uppersemiconductor layer 403 of a Si film due to epitaxial growth, on which asecond BN film 426 also acting as an insulating layer is depositedfalling under the regulation of crystallographic axis of the first uppersemiconductor layer 403. Finally, a second upper semiconductor layer 404is deposited on the second BN film 426 which is regulated by thecrystallographic axis of the second BN film 426 to be epitaxially grown.

It will be noted that the semiconductor device shown in FIG. 5 isadapted to substantially improve the integration to a level sufficientto carry out the high density packaging because there are provided threemonocrystalline semiconductor layers into each of which a semiconductorelement is adapted to be incorporated. Also, the semiconductor devicehas an advantage that because the BN films 416 and 426 each areeffectively preferentially orientated to the C-axis even with respect toa semiconductor layer having roughness somewhat formed on the surfacethereof due to the incorporation of a semiconductor element thereinto,the formation or incorporation of a semiconductor element may be carriedout with respect to either surface of each semiconductor layer 403 or404.

FIG. 6 shows a fifth semiconductor device including a BN film of thepresent invention acting as an insulating layer, which has been preparedin view of the foregoing. More particularly, in the device, a Si devicesuch as an integrated circuit or the like is incorporated into the uppersurface side 502a of a monocrystalline Si semiconductor wafer to form alower semiconductor layer 502, and a BN film 516 of the presentinvention is deposited on the lower semiconductor layer 502 in such amanner as described above. Further, on the BN film 516 is deposited anupper semiconductor layer 503 of a monocrystalline Si. It is of coursethat in the device an integrated circuit or the like may be incorporatedinto the upper semiconductor layer 503.

The high integration of a semiconductor integrated circuit in recentyears requires to give a careful consideration to an improvement in heatdissipation of semiconductor devices such as a semiconductor element forlarge power, for example, a power transistor, a linear integratedcircuit for an audio equipment and the like. Such an improvement in heatdissipation characteristics of a semiconductor device renders a furtherincrease in integration of the device possible.

FIG. 7 illustrates a sixth semiconductor device having a BN film of thepresent invention provided therein, which has been prepared in view ofthe foregoing while taking notice of the fact that the heat dissipationcharacteristics of a semiconductor device which is highly integrated andpackaged with high density can be significantly improved by depositing ametallic film for heat dissipation such as an A1 film on a BN film ofthe present invention to more effectively improve an excellent heatconductivity of the BN film to efficiently discharge heat through themetallic film to the exterior or carry out shielding between multi-layerelements by means of the metallic film, depositing a second BN film ofthe present invention on the metallic film, and forming anothersemiconductor layer on the second BN film.

More particularly, the semiconductor device of FIG. 7 comprises a lowersemiconductor layer 602 having a semiconductor element incorporatedthereinto in the substantially same manner as the device of FIG. 6, afirst BN film 616 deposited on the lower semiconductor layer 602 to actas an insulating layer, a heat dissipation plate 605 of a metallic ormagnetic material deposited on the first BN film 616, a second BN film626 deposited on the heat dissipation plate 605 to act as an insulatinglayer, and an upper semiconductor layer 604 formed on the second BN film626. The upper semiconductor layer 604 is formed therein with anintegrated circuit or the like, and the electrical connection betweenthe upper and lower semiconductor layers 604 and 602 is carried out bymeans of wirings 607 inserted through small through-holes 606 formed atthe heat dissipation plate 605.

In the semiconductor device of FIG. 7, the crystalline BN films 616 and626 have not only excellent heat conduction characteristics but ananisotropy in heat conduction characteristics. The thermal conductivityof each BN film in the direction parallel to the crystallographic axisis about three times as much as that in the perpendicular to the axis,so that the semiconductor device may exhibit an effect of enhancing heatdissipation from the upper and lower semiconductor layers 604 and 602 tothe heat dissipation plate 605. This results in a semiconductor devicefor large power such as a power transistor or a semiconductor devicepackaged with high density which is provided therein with suchcrystalline BN film acting as an insulating layer carrying out heatdissipation with good efficiency. In this instance, the formation ofheat dissipation plate 605 by a metallic material exhibits anelectrostatic shielding effect, whereas that by a magnetic materialprovides a magnetic shielding effect.

In each of the semiconductor devices described above, it is mostconvenient that the formation of BN film or semiconductor layer of goodquality is carried out according to the ICB process proposed by theinventors. The ICB process effectively exhibits a surface migrationeffect that clusters separate into individual atoms upon impingement ona substrate to diffuse on the surface of the substrate, an effect ofeffectively acting on the formation or coalescence of seeds of crystalgrowth at the initial stage at which ions contained in clusters or atomsform a film, a surface cleaning effect due to kinetic energy imparted toa vapor stream, an ion implantation effect and the like, resulting in afilm being formed which has good quality, high adhesion to a substrateand a good flatness from the viewpoint of crystallography. Also, the ICBprocess has a high degree of freedom with respect to film depositionconditions such as, for example, the difference in pressure between theinterior and exterior of a crucible, ionization in an ionization chamberand a degree of such ionization, an electrical field for accelerationand the like, so that the control of crystal structure of a film to beformed may be facilitated to permit a monocrystalline film of goodquality to be prepared depending upon desired characteristics.

Also, in the devices shown in FIGS. 2 to 7, a temperature of thesubstrate at the time of forming the BN film can be set as desiredbetween a room temperature and 500° to 600° C. Thus, there is no fear ofcausing impurity concentration of the semiconductor element which hasbeen already incorporated in the semiconductor layer to be inbalanced.This is helpful to form a three-dimensional semiconductor device.

Now, results in experiments on a BN film according to the presentinvention will be described with reference to FIGS. 8 to 10.

FIG. 8 shows a result of X-ray diffraction of a BN film formed on asubstrate of a monocrystalline Si semiconductor layer in thesubstantially same manner as described above according to the ICBprocess under the conditions that acceleration voltage Va across anacceleration electrode and electronic current for ionization Ie are setat 0.5 kV and 100 mA, respectively, wherein the axis of abscissasindicates a reflection angle of X-ray (20).

As shown in FIG. 8, a strong reflection peak appears from the directionof the C-axis (002) of the BN film having a wurtzite structure at theposition where the X-ray reflection angle (2θ) is 12.2°. This clearlyindicates that the formed BN film is not amorphous but is preferentiallyorientated to the specified crystallographic axis along the C-axis(002).

Also, a result (not shown) of another X-ray diffraction on a BN filmformed on an amorphous substrate of glass indicates that a BN film canbe effectively formed due to preferential orientation to the C-axis(002) also on glass which is amorphous, although a reflection peak fromthe C-axis (002) is weak as compared with that of a BN film formed onthe above-mentioned monocrystalline Si semiconductor layer under thesame conditions.

Further, a misfit in lattice constant between the monocrystalline Sisemiconductor layer and the BN film is as small as 5.9%. Thus, it willbe noted that when Si is grown on the BN film preferentially orientatedto the C-axis, it is substantially completely and readily epitaxiallygrown utilizing the crystallographic axis of the BN film as seeds ofcrystal growth to form a monocrystalline Si film sufficient toincorporate a semiconductor element thereinto.

The determination of infrared reflectance spectral characteristics of aBN film formed on a glass substrate (amorphous) under the samedisposition conditions as in FIG. 8 shows the appearance of a peak dueto a stretching vibration of the bond between nitrogen atom and boronatom at a wavenumber of 1380 cm⁻¹ as well as that due to a stretchingvibration of the bond between oxygen atom and hydrogen atom or betweennitrogen atom and hydrogen atom. The peak has a relatively small halfband width. These facts clearly indicate the formation of a BN film ofgood quality from the viewpoint of crystallography although it somewhatcontains hydrogen, oxygen or a combination thereof.

FIG. 9 shows electrical conductivity of a BN film measured near a roomtemperature (T=300 K) which was formed on a substrate under the samedeposition conditions as in FIG. 8, wherein the axis of abscissasindicates temperature (1000/T) and the axis of ordinates indicatesconductivity.

As shown in FIG. 9, the BN film has an extremely low conductivity ofabout 10⁻⁹ Ω⁻¹.cm⁻¹ to 10⁻¹¹ Ω⁻¹.cm⁻¹, and this clearly shows that itexhibits excellent electrical insulating properties. Accordingly, theuse of such BN film in a manner to be interposed between layers of suchan integration-settled semiconductor IC device as shown in FIGS. 2 to 7permits the layers to be positively insulated from each other.

Further, thermal conductivity of a BN film of the C-axis preferentialorientation formed on an amorphous substrate of glass under the samedeposition conditions as in FIGS. 8 and 9 was measured utilizing athermal pulse method. In the experiment, it was difficult to measureconductivity of the BN film itself because it is of the order of micronin thickness, however, the BN film deposited on the glass substrate wasmeasured to have conductivity of 0.116 W/cm.deg in the areal directionor the direction perpendicular to the growth axis, which issubstantially equal to conductivity of stainless steel. As describedabove, in the crystalline BN film, the conductivity along the growthaxis is about three times as much as that in the direction perpendicularthereto.

Accordingly, as shown in FIG. 10, when a semiconductor layer 100 isdeposited thereon a BN film in such a manner to be orientated to apredetermined axis or the C-axis along the heat dissipation direction ofthe layer 100 as in the semiconductor device of FIG. 7 and further aheat dissipation plate 800 is formed on the BN film 700, heatdissipation is effectively carried out because heat generated by thesemiconductor layer 600 is efficiently transmitted through the BN film700 to the heat dissipation plate 800. This enhances the heatdissipation of a semiconductor device for large power such as a powertransistor and realizes the integration and high density packaging of asemiconductor device.

The BN film and the process for preparing the same according to thepresent invention are not restricted to such specific forms as describedhereinbefore. For example, a material to be charged in the crucible maybe boron sulfide. Alternatively, N₂ gas or NH₃ gas may be used as areactive gas to be introduced in the vacuum casing.

As can be seen from the foregoing, the BN film of the present inventionis adapted to be formed in a manner to be preferentially orientated to apredetermined axis thereof on the surface of a substrate. The present BNfilm, when a substrate is crystalline such as a monocrystalline Sisemiconductor layer, is epitaxially grown along the crystallographicaxis of the layer, whereas, when a substrate is amorphous such as glass,it is grown along the C-axis (002).

Thus, the BN film according to the present invention is capable ofserving as seeds for allowing a semiconductor layer to be epitaxiallygrown thereon, into which a semiconductor element is incorporated, andbeing used as an intermediate insulating layer for a three-dimensionalsemiconductor device and the like even in the form of a thin layerbecause of its high electrical insulating properties so that it maypromote the high integration and high density packaging of athree-dimensional semiconductor device. Also, the BN film of theinvention contributes to the manufacture of a semiconductor devicebecause it can use inexpensive glass as a substrate.

In addition, in the BN film according to the present invention, thethermal conductivity along a predetermined axis thereof is about threetimes as much as a prior art amorphous BN film. Accordingly, theinterposition of the present BN film between a semiconductor layer and aheat dissipation plate formed of metal or the like and acting as a heatsink provides an excellent heat dissipation capacity while keeping highelectrical insulating properties. This effectively eliminatesinconvenience such as insulation of the whole heat dissipation plate ina semiconductor device for large power generating a large amount ofheat, to thereby carry out high density packaging of the semiconductordevice.

Further, the positioning of the present BN film between a semiconductorlayer and a heat dissipation plate in a semiconductor device to carryout heat conduction along, in particular, a predetermined axis or theC-axis of the BN film exhibits a more effective heat dissipation effect.

Furthermore, the process of preparing a BN film according to the presentinvention is practiced in a manner to heat and vaporize boride chargedin a closedtype crucible having at least one injection nozzle, eject thevapor of boron through the nozzle into a nitrogen atmosphere below 10⁻²Torr in pressure to ionize at least a part thereof, and impinge theionized vapor on the surface of a substrate together with nitrogen.Thus, it facilitates the preparation of a BN film and the presentprocess can form a BN film at a relatively low temperature so that a BNfilm easy to be handled may be formed which has a high adhesion withrespect to a substrate and a good surface flatness and never adverselyaffects the impurity profile of a semiconductor element which has beenalready incorporated into a semiconductor device.

As many apparently widely different embodiments of this invention may bemade without departing from the spirit and scope thereof, it is to beunderstood that the invention is not limited to the specific embodimentsthereof except as defined in the appended claims.

What is claimed as new and desired to be secured by Letters Patent ofthe United States is:
 1. A process for preparing a boron nitride filmcomprising the steps of:heating boride charged in a closed-type cruciblehaving at least one injection nozzle to form boron vapor; ejecting saidboron vapor through said nozzle into a nitrogen atmosphere having apressure below 10⁻² Torr and ionizing at least a part thereof; andimpinging said ionized boron vapor on the surface of a substratetogether with nitrogen to form a boron nitride film on said substrate.2. A process as defined in claim 1, wherein said boride is boron oxide.3. A process as defined in claim 1, wherein said boride is boronsulfide.
 4. A process as defined in claim 1, wherein said substrate isformed of an amorphous material.
 5. A process as defined in claim 1,wherein said substrate is formed of a monocrystalline material.